Matrix phase interpolator for phase locked loop

ABSTRACT

Generating a composite interpolated phase-error signal for clock phase adjustment of a local oscillator by forming a summation of weighted phase-error signals generated using a matrix of partial phase comparators, each of which compare a phase of the local oscillator with a corresponding phase of a reference clock.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No. 15/602,080, filed May 22, 2017, naming Ali Hormati and Armin Tajalli, entitled “Data-Driven Phase Detector Element for Phase Locked Loop”, which claims the benefit of U.S. Provisional Application No. 62/395,993, filed Sep. 16, 2016, entitled “Matrix Phase Detector Element for Phase Locked Loop”, all of which are hereby incorporated herein by reference in their entirety for all purposes.

REFERENCES

The following prior applications are herein incorporated by reference in their entirety for all purposes:

U.S. Patent Publication 2011/0268225 of application Ser. No. 12/784,414, filed May 20, 2010, naming Harm Cronie and Amin Shokrollahi, entitled “Orthogonal Differential Vector Signaling” (hereinafter “Cronie I”).

U.S. Patent Publication 2011/0302478 of application Ser. No. 12/982,777, filed Dec. 30, 2010, naming Harm Cronie and Amin Shokrollahi, entitled “Power and Pin Efficient Chip-to-Chip Communications with Common-Mode Resilience and SSO Resilience” (hereinafter “Cronie II”).

U.S. patent application Ser. No. 13/030,027, filed Feb. 17, 2011, naming Harm Cronie, Amin Shokrollahi and Armin Tajalli, entitled “Methods and Systems for Noise Resilient, Pin-Efficient and Low Power Communications with Sparse Signaling Codes” (hereinafter “Cronie III”).

U.S. patent application Ser. No. 13/176,657, filed Jul. 5, 2011, naming Harm Cronie and Amin Shokrollahi, entitled “Methods and Systems for Low-power and Pin-efficient Communications with Superposition Signaling Codes” (hereinafter “Cronie IV”).

U.S. patent application Ser. No. 13/542599, filed Jul. 5, 2012, naming Armin Tajalli, Harm Cronie, and Amin Shokrollahi entitled “Methods and Circuits for Efficient Processing and Detection of Balanced Codes” (hereafter called “Tajalli I”.)

U.S. patent application Ser. No. 13/842,740, filed Mar. 15, 2013, naming Brian Holden, Amin Shokrollahi and Anant Singh, entitled “Methods and Systems for Skew Tolerance in and Advanced Detectors for Vector Signaling Codes for Chip-to-Chip Communication”, hereinafter identified as [Holden I];

U.S. Provisional Patent Application No. 61/946,574, filed Feb. 28, 2014, naming Amin Shokrollahi, Brian Holden, and Richard Simpson, entitled “Clock Embedded Vector Signaling Codes”, hereinafter identified as [Shokrollahi I].

U.S. patent application Ser. No. 14/612,241, filed Aug. 4, 2015, naming Amin Shokrollahi, Ali Hormati, and Roger Ulrich, entitled “Method and Apparatus for Low Power Chip-to-Chip Communications with Constrained ISI Ratio”, hereinafter identified as [Shokrollahi II].

U.S. patent application Ser. No. 13/895,206, filed May 15, 2013, naming Roger Ulrich and Peter Hunt, entitled “Circuits for Efficient Detection of Vector Signaling Codes for Chip-to-Chip Communications using Sums of Differences”, hereinafter identified as [Ulrich I].

U.S. patent application Ser. No. 14/816,896, filed Aug. 3, 2015, naming Brian Holden and Amin Shokrollahi, entitled “Orthogonal Differential Vector Signaling Codes with Embedded Clock”, hereinafter identified as [Holden II].

U.S. patent application Ser. No. 14/926,958, filed Oct. 29, 2015, naming Richard Simpson, Andrew Stewart, and Ali Hormati, entitled “Clock Data Alignment System for Vector Signaling Code Communications Link”, hereinafter identified as [Stewart I].

U.S. patent application Ser. No. 14/925,686, filed Oct. 28, 2015, naming Armin Tajalli, entitled “Advanced Phase Interpolator”, hereinafter identified as [Tajalli II].

U.S. Provisional Patent Application No. 62/286,717, filed Jan. 25, 2016, naming Armin Tajalli, entitled “Voltage Sampler Driver with Enhanced High-Frequency Gain”, hereinafter identified as [Tajalli III].

U.S. Provisional Patent Application No. 62/288,717, filed Apr. 22, 2016, naming Armin Tajalli, entitled “High Performance Phase Locked Loop”, hereinafter identified as [Tajalli IV].

The following additional references to prior art have been cited in this application:

U.S. Pat. No. 6,509,773, filed Apr. 30, 2001 by Buchwald et al., entitled “Phase interpolator device and method” (hereafter called [Buchwald].

“Linear phase detection using two-phase latch”, A. Tajalli, et al., IEE Electronic Letters, 2003, (hereafter called [Tajalli V].)

“A Low-Jitter Low-Phase-Noise 10-GHz Sub-Harmonically Injection-Locked PLL With Self-Aligned DLL in 65-nm CMOS Technology”, Hong-Yeh Chang, Yen-Liang Yeh, Yu-Cheng Liu, Meng-Han Li, and Kevin Chen, IEEE Transactions on Microwave Theory and Techniques, Vol 62, No. 3, March 2014 pp. 543-555, (hereafter called [Chang et al.])

“Low Phase Noise 77-GHz Fractional-N PLL with DLL-based Reference Frequency Multiplier for FMCW Radars”, Herman Jalli Ng, Rainer Stuhlberger, Linus Maurer, Thomas Sailer, and Andreas Stelzer, Proceedings of the 6th European Microwave Integrated Circuits Conference, 10-11 Oct. 2011, pp. 196-199, (hereafter called [Ng et al.])

“Design of Noise-Robust Clock and Data Recovery using an Adaptive-Bandwidth Mixed PLL/DLL”, Han-Yuan Tan, Doctoral Thesis, Harvard University November 2006, (hereafter called [Tan]).

U.S. Pat. No. 7,492,850, filed Aug. 31, 2005 by Christian Ivo Menolfi and Thomas Helmut Toifl, entitled “Phase locked loop apparatus with adjustable phase shift” (hereafter called [Menolfi].)

“A Calibration-Free Fractional-N Ring PLL Using Hybrid Phase/Current-Mode Phase Interpolation Method”, by Romesh Kumar Nandwana et al, IEEE Journal of Solid-State Circuits Vol. 50, No. 4, April 2015, ppg. 882-895, (hereafter called [Nandwana].)

FIELD OF THE INVENTION

The present invention relates to communications systems circuits generally, and more particularly to obtaining a stable, correctly phased receiver clock signal from a high-speed multi-wire interface used for chip-to-chip communication.

BACKGROUND

In modern digital systems, digital information has to be processed in a reliable and efficient way. In this context, digital information is to be understood as information available in discrete, i.e., discontinuous values. Bits, collection of bits, but also numbers from a finite set can be used to represent digital information.

In most chip-to-chip, or device-to-device communication systems, communication takes place over a plurality of wires to increase the aggregate bandwidth. A single or pair of these wires may be referred to as a channel or link and multiple channels create a communication bus between the electronic components. At the physical circuitry level, in chip-to-chip communication systems, buses are typically made of electrical conductors in the package between chips and motherboards, on printed circuit boards (“PCBs”) boards or in cables and connectors between PCBs. In high frequency applications, microstrip or stripline PCB traces may be used.

Common methods for transmitting signals over bus wires include single-ended and differential signaling methods. In applications requiring high speed communications, those methods can be further optimized in terms of power consumption and pin-efficiency, especially in high-speed communications. More recently, vector signaling methods have been proposed to further optimize the trade-offs between power consumption, pin efficiency and noise robustness of chip-to-chip communication systems. In those vector signaling systems, digital information at the transmitter is transformed into a different representation space in the form of a vector codeword that is chosen in order to optimize the power consumption, pin-efficiency and speed trade-offs based on the transmission channel properties and communication system design constraints. Herein, this process is referred to as “encoding”. The encoded codeword is communicated as a group of signals from the transmitter to one or more receivers. At a receiver, the received signals corresponding to the codeword are transformed back into the original digital information representation space. Herein, this process is referred to as “decoding”.

Regardless of the encoding method used, the received signals presented to the receiving device must be sampled (or their signal value otherwise recorded) at intervals best representing the original transmitted values, regardless of transmission channel delays, interference, and noise. This Clock and Data Recovery (CDR) not only must determine the appropriate sample timing, but must continue to do so continuously, providing dynamic compensation for varying signal propagation conditions.

Many known CDR systems utilize a Phase-Locked Loop (PLL) or Delay-Locked Loop (DLL) to synthesize a local receive clock having an appropriate frequency and phase for accurate receive data sampling.

BRIEF DESCRIPTION

To reliably detect the data values transmitted over a communications system, a receiver must accurately measure the received signal value amplitudes at carefully selected times. Various methods are known to facilitate such receive measurements, including reception of one or more dedicated clock signals associated with the transmitted data stream, extraction of clock signals embedded within the transmitted data stream, and synthesis of a local receive clock from known attributes of the communicated data stream.

In general, the receiver embodiments of such timing methods are described as Clock-Data Recovery (CDR), often based on Phase-Lock Loop (PLL) or Delay-Locked Loop (DLL) synthesis of a local receive clock having the desired frequency and phase characteristics.

In both PLL and DLL embodiments, a Phase Detector compares the relative phase (and in some variations, the relative frequency) of a received reference signal and a local clock signal to produce an error signal, which is subsequently used to correct the phase and/or frequency of the local clock source and thus minimize the error. As this feedback loop behavior will lead to a given PLL embodiment producing a fixed phase relationship (as examples, 0 degrees or 90 degrees of phase offset) between the reference signal and the local clock, an additional fixed or variable phase adjustment is often introduced to permit the phase offset to be set to a different desired value (as one example, 45 degrees of phase offset) to facilitate receiver data detection.

Methods and systems are described for receiving, at a data-driven phase comparator circuit, a plurality of data signals in parallel from a plurality of multi-input comparators (MICs) connected to a multi-wire bus, wherein at least one MIC is connected to at least three wires of the multi-wire bus, and one or more phases of a local oscillator signal, the data-driven phase comparator circuit comprising a plurality of partial phase comparators, generating a plurality of partial phase-error signals using the partial phase comparators, each partial phase-error signal generated by receiving (i) a corresponding phase of the local oscillator signal and (ii) a corresponding data signal of the plurality of data signals and responsive to a determination that a transition occurred in the corresponding data signal, generating the partial phase-error signal based on a comparison of the corresponding phase of the local oscillator signal and the corresponding data signal, and generating a composite phase-error signal by summing the plurality of partial phase error signals, the composite phase-error signal for setting a local oscillator generating the one or more phases of the local oscillator signal in a lock condition.

Embodiments are described in which the Phase Detection and phase adjustment elements are combined, leading to lower circuit node capacitance and reduced circuit delays, these improvements in turn enabling increased loop stability and improved PLL lock characteristics, including increased loop lock bandwidth leading to lower clock jitter and improved power supply noise rejection.

Embodiments are also described in which a Delay-Locked Loop is used to convert the received reference clock signal into multiple reference clock phases, converting the PLL phase comparison operation into multiple comparisons made between a reference clock phase and a local clock phase. A summation or weighted summation of the multiple comparison results is then used as the error feedback signal for the PLL. A further embodiment is described in which multiple comparisons are made between a single received reference clock phase and multiple local clock phases, with the weighted sum of the multiple comparison results used as the error feedback term for the PLL. In at least one such further embodiment, said weighted sums comprise a two dimensional time domain filter.

BRIEF DESCRIPTION OF FIGURES

FIG. 1 is a block diagram of one embodiment capable of encoding and transmitting five data bits and a clock on an eight wire communications channel.

FIG. 2 is a block diagram of one embodiment of a receiver compatible with the transmitter of FIG. 1.

FIG. 3 is a block diagram detailing one embodiment of the clock recovery circuit used by the receiver of FIG. 2.

FIGS. 4A, 4B, and 4C show three Phase Detector embodiments suitable for use in a Phase Locked Loop element of a clock recovery circuit.

FIG. 5 is a schematic diagram of one embodiment integrating an XOR phase detector and clock phase interpolator.

FIG. 6A is a schematic diagram of a clocked data latch and FIG. 6B is a schematic diagram of a further embodiment of a clocked data latch integrating a clock phase interpolator.

FIGS. 7A and 7B are schematic diagrams of an embodiment integrating a state machine phase detector and clock phase interpolators.

FIG. 8 is a schematic diagram of one embodiment of a charge pump suitable for further integration with a phase comparator embodiment.

FIG. 9 is a block diagram of a further embodiment in which multiple phases of a reference clock are compared with multiple local clock phases.

FIG. 10 is a block diagram of a further embodiment in which multiple comparisons are made between a single reference clock and multiple local clock phases.

FIG. 11A is a weighted XOR phase detector, in accordance with some embodiments.

FIG. 11B is a block diagram of one embodiment of a matrix phase comparison of M reference phases and N local clock phases.

FIGS. 12A and 12B are block diagrams of an alternate embodiment of the integrated phase detector and phase interpolator of FIG. 5.

FIG. 13A is a timing diagram for a folded phase detector, in accordance with some embodiments.

FIG. 13B is timing diagram illustrating a reverse clipping effect, in accordance with some embodiments.

FIGS. 14A and 14B are timing diagrams for an array-XOR phase detector and single-XOR phase detector, respectively, in accordance with some embodiments.

FIG. 15 illustrates XOR-based phase comparator and correction signals applied to a loop filter, in accordance with some embodiments.

FIG. 16 illustrates time-domain error signals produced by a row-based phase comparator in accordance with some embodiments.

FIG. 17 illustrates phase interpolation based on interpolation a lock point between two consecutive diagonals of a two dimensional phase comparator array, in accordance with some embodiments.

FIG. 18 illustrates improved resolution of a phase interpolator in a diagonal multi-phase detector structure, in accordance with some embodiments.

FIGS. 19A-19D illustrate various partial phase comparator architectures, in accordance with some embodiments.

FIG. 20 illustrates an exemplary XOR phase comparator architecture, in accordance with some embodiments.

FIG. 21 illustrates a timing diagram of output currents lout of an XOR phase comparator shown in FIG. 20, in accordance with some embodiments.

FIG. 22 is a simulated phase comparator response, in accordance with some embodiments.

FIG. 23 is a simulation of a phase-locked loop bandwidth, in accordance with some embodiments.

FIG. 24 is a block diagram of an oversampled multi-phase feedback phase-locked loop (MPLL) in accordance with some embodiments.

FIG. 25 is a block diagram of a receiver, in accordance with some embodiments.

FIG. 26 is a block diagram of a clock recovery circuit operating on detected data signals, in accordance with some embodiments.

FIG. 27 is a block diagram of a matrix phase comparator operating on detected data signals, in accordance with some embodiments.

FIG. 28 is a flowchart of a method, in accordance with some embodiments.

FIG. 29 is a block diagram of an edge-triggered bang-bang phase detector.

FIG. 30 is a block diagram of a linear edge-triggered phase detector.

FIG. 31 is a wave-form associated with the linear edge-triggered phase detector of FIG. 30.

DETAILED DESCRIPTION

As described in [Cronie I], [Cronie II], [Cronie III] and [Cronie IV], vector signaling codes may be used to produce extremely high bandwidth data communications links, such as between two integrated circuit devices in a system. As illustrated by the embodiment of FIG. 1, multiple data communications channels transmit symbols of the vector signaling code, acting together to communicate codewords of the vector signaling code. Depending on the particular vector signaling code used, the number of channels comprising a communications link may range from two to eight or more, and may also communicate one or more clock signals on separate communications channels or as sub-channel components of the vector signaling code. In the example of FIG. 1, communication link 120 is illustrated as being composed of eight wires 125, collectively communicating five data values 100 and one clock 105 between transmitter 110 and receiver 130.

Individual symbols, e.g. transmissions on any single communications channel, may utilize multiple signal levels, often three or more. Operation at channel rates exceeding 10 Gbps may further complicate receive behavior by requiring deeply pipelined or parallelized signal processing, precluding reception methods that include the previous received value to be known as the current value is being received.

Embodiments described herein can also be applied to prior art permutation sorting methods not covered by the vector processing methods of [Cronie II], [Cronie III], [Cronie IV], and/or [Tajalli I]. More generally, embodiments may apply to any communication or storage methods requiring coordination of multiple channels or elements of the channel to produce a coherent aggregate result.

Receiver Data Detection

To provide context for the following examples, one typical high-speed receiver embodiment [Stewart I] is used for illustrative purposes, without limitation.

As illustrated in FIG. 2, the example data receiver includes eight identical Continuous Time Linear Equalization (CTLE) stages 210 operating on the signals received on the eight wires, previously shown as 120 in FIG. 1.

As described in [Tajalli I], [Holden I] and [Ulrich I], vector signaling codes may be efficiently detected by linearly combining sets of input signals using Multi-Input comparators or mixers (MIC). For the 5b6w code used by the example receiver, five such mixers acting on weighted subsets of the six received data input signals will detect the five data bits without need of further decoding. One additional mixer acting on combinations of the two received clock signals will similarly detect the clock signal. In FIG. 2, this set of six MIC mixers 220 operate on the received and equalized signals to produce detected signals MIC0-MIC5.

Because of the high data rates involved, multiple parallel phases of receive processing are shown in the example receiver. In one embodiment, the five detected data signals MIC0-MIC4 are processed in four parallel phases of receive data processing, each phase 230 including five data samplers and subsequent buffering, followed by recombination of the four phase outputs into a received data stream, shown in FIG. 2 as being performed by multiplexers 240.

Clock Recovery circuits (also known in the art as Clock Data Recovery or CDR) support such sampling measurements by extracting timing information, either from the data lines themselves or from dedicated clock signal inputs, and utilize that extracted information to generate clock signals to control the time interval used by the data line sampling device(s). The actual clock extraction may be performed using well known circuits such as a Phase Locked Loop (PLL) or Delay Locked Loop (DLL), which in their operation may also generate higher frequency internal clocks, multiple clock phases, etc. in support of receiver operation. In the embodiment of FIG. 2, the detected clock signal is obtained at MIC5 and processed 300 to extract properly timed sampling clocks for the four data phases.

Other embodiments may forgo the dedicated wires used to communicate a separate clock signal, and instead be configured to have the receiver extract a clock from transitions occurring on the data lines themselves. As is well understood in the art, successful application of this technique requires a sufficiently large transition density on the data lines (which may be alternatively interpreted as requiring a sufficiently small interval between transitions,) and sufficient free-running frequency stability within the PLL to maintain accurate data sample timing during non-transition intervals. [Shokrollahi I] describes suitable vector signaling codes having such guaranteed transition density. Alternatively, known art transition-enforcing encoding such as the commonly utilized 8b10b and 64b66b codes may be applied to the data stream (or data streams of individual subchannels) to ensure subsequent transitions in the vector signal code elements on each subchannel to insure a guaranteed minimum transition density at the receiver.

PLL Overview

Phase Locked Loops are well represented in the literature. A typical PLL is composed of a phase detector that compares an external reference signal to an internal clock signal, a low pass filter that smooths the resulting error value to produce a clock control signal, and a variable frequency clock source (typically, a Voltage Controlled Oscillator or VCO) controlled by the smoothed error value, producing the internal clock signal presented to the phase detector. In a well-know variation, such a PLL design may incorporate a clock frequency divider between the VCO and the phase detector, allowing a higher-frequency clock output to be phase locked to a lower-frequency reference signal.

In an alternative embodiment, the variable frequency clock source is replaced by a variable delay element, its (optionally multiple tapped) outputs thus representing one or more successive time-delayed versions of the original input signal rather than successive cycles of an oscillator to be phase compared to the reference input signal. For the purposes of this document, such Delay Locked Loops (DLL) are considered functionally equivalent to a PLL in such an application, particularly in regard to composed elements of phase detector, phase interpolator, and charge pump.

Numerous forms of phase detectors are known to the art. A simple XOR gate as in FIG. 4A may be used to compare, as a non-limiting example, two square wave signals. One familiar with the art will observe that such a digital XOR output will be a variable-duty-cycle waveform which, when low pass filtered into an analog error signal, results in a proportional error signal centered in its analog signal range when the two input signals have a 90 degree phase offset relationship.

The more complex state machine phase detector of FIG. 4B is composed of two edge-triggered latches clocked respectively by the reference and internal clock signals, with the first received clock edge initiating an output signal on one of the “early” or “late” outputs. Either output becoming active will subsequently cause the latches to reset in anticipation of the next comparison interval. Alternative embodiments may incorporate a timing delay in this reset path to provide additional control of the reset pulse timing, as indicated by the “Hold” signal. The “late” and “early” phase comparison outputs are typically accepted as “pump up” and “pump down” inputs to a charge pump, the output of which is the analog error value. That is, a pump up signal may turn on a first transistor circuit that provides charge to capacitor thereby increasing the analog voltage, while a pump down signal may turn on a second transistor circuit that removes charge from a capacitor, thereby reducing the voltage. A zero degree phase offset between the two input clock signals will thus leave the analog error value unchanged and the PLL in a stable locked condition. A number of equivalent state machine phase detector embodiments are known in the art, and are equally applicable in this application, without implication of limitation. Some state machine embodiments may be sensitive to both phase and frequency differences between the input signals, facilitating more rapid PLL lock acquisition on startup.

As shown in FIG. 4C, a simple edge-clocked “D” flip-flop may also be used as a phase detector. At each rising local clock edge (CkPLL), the D input samples the state of the (in this example, square wave) reference input (CkRef); if it is high (e.g. it has already transitioned,) the Q output is high indicating the reference is “early”, if it is low (e.g. it has not yet transitioned,) Q is low indicating the reference is “late”. This so-called “bang/bang” phase detector provides a less nuanced error result than the previous example, thus may utilize more sophisticated filtering to obtain loop stability.

As will be recognized by those familiar with the art, comparable functional operation may be obtained regardless of the phase detector type incorporated in a PLL design, thus to first approximation phase detector choice is not limiting. Secondary design behaviors, including lock time, stability, power consumption, etc. must also be considered as part of the design process.

Receiver Clock Recovery

The example receiver utilizes a PLL embodiment as shown in FIG. 3. This PLL accepts the received clock signal R5 as the reference to which its clocks will be phased locked. In some embodiments, logic level shift 310 is used if appropriate to interface between the signal levels provided by the detecting MIC and the preferred phase comparator input levels. Phase Comparator 320 compares the reference clock to an internal clock derived from the VCO, producing an output which is low pass filtered to provide an Error value which subsequently corrects the operating frequency of VCO 340. In some embodiments, the outputs of Phase Comparator 320 is a digital waveform requiring conversion to an analog error signal, either through implicit or explicit digital to analog conversion, or by use of an interface element such as a charge pump. Some embodiments may combine such conversion with all or part of the low pass filtering operation, as one example offered without limitation, by the digital filtering behavior shown by the switching action of a charge pump directed by digital control signals generating an analog signal output.

In one embodiment, a ring oscillator 340 composed of a sequence of identical gates in a closed loop is used as the internal Voltage Controlled Oscillator (VCO) timing source for the PLL. The VCO frequency is varied by analog adjustment of at least one of: gate propagation delay, inter-gate rise and fall time, and gate switching threshold within the ring oscillator. This may be implemented via switched capacitor banks, where a digital control signal is applied to selective place capacitive elements in parallel and/or series combinations to alter an RC time constant, as one non-limiting example. Still further, a current source that drives a gate of the ring oscillator may be increased or decreased to alter the output switching rise-time/fall-time, and thereby adjust the effective delay. Outputs taken at equal intervals (i.e. separated by equal numbers of ring oscillator gates) along the sequence of gates comprising the ring oscillator provide the four data phase sampling clocks, herein identified as the 0, 90, 180, and 270 degree clocks.

In one embodiment, the ring oscillator is composed of eight identical sets of logic gates (e.g., a set of inverter circuits), thus the phase difference from one such set to the next is 45 degrees. In this embodiment, the 0, 90, 180, and 270 degree outputs may be obtained, as examples, from the second, fourth, sixth, and eighth outputs. As these clocks are cyclical, the final tap may be considered as logically adjacent to the initial tap, a 0 degree and a 360 degree phase offset being equivalent. As many variations of such designs are known in the art, neither the number of elements in the ring oscillator nor the specific taps at which particular outputs are made should be construed as implying a limitation. As one example, the location of the 0 degree tap is arbitrary, as one familiar with the art will recognize that normal PLL behavior will phase align the ring oscillator with the external phase reference regardless of its initial phase. Similarly, equivalent designs may be obtained in which the output clock phases do not have square wave duty cycles; as one example being produced by the action of AND or OR gates with inputs from different tap locations. In the example receiver, it is desired that the VCO operate at a multiple of the received reference clock frequency, thus Frequency Divider 350 divides the VCO outputs by a comparable amount prior to the Phase Detector. In one embodiment, binary (factor of two) dividers are used at 350 to obtain the correct sampling clock rate. In another embodiment, no divider is utilized and the VCO outputs are presented to the phase interpolator directly.

Each of the four phases of sampling clocks is appropriately timed to sample received data for one of the four parallel processing phases. In particular, internal clock ph000 is aligned to optimally trigger data samplers in the phase0 phase of processing, clock ph090 in phase1, clock ph180 in phase2, and clock ph270 in phase3.

To allow the overall phase of the locked PLL signals to be offset from the reference clock input phase, the local clock output presented to the phase comparator is obtained from phase interpolator 360, the output phase of which is controllably intermediate between its input clock phases. Thus, the PLL may lock with its fixed phase relationship, while the internal clock signals obtained from ring oscillator 340 will be offset from that fixed phase by the phase delay amount introduced by phase interpolator 350, as controlled by signal Phase offset correction generated by clock/data phase control logic 370. Phase interpolators are known in the art, examples being provided by [Buchwald I] and [Tajalli II].

In one embodiment, phase interpolator 360 receives multiple clock phases from the ring oscillator 340 having 90 degree phase differences. Said phase interpolator may be controlled to select two adjacent clock input phases and then to interpolate between them so as to produce an output at a chosen phase offset between those selected two values. For purposes of description, it may be assumed that a phase detector design is used which drives the PLL to lock with a zero phase differential between the two phase detector inputs. Thus, continuing the example, applying the 0 and 90 degree clock phases as inputs to the phase interpolator allows adjustment such that the PLL leads the reference clock input by between 0 and 90 degrees.

It will be apparent that equivalent results with comparable phase offsets may be obtained using other pairs of degree clocks and/or other phase detector designs, which as previously described may lock with different phase differentials than that of the present example. Thus neither the particular phase clocks chosen nor the particular phase detector design described herein are limiting.

In the known art, [Nandwana] describes a Fractional-N clock multiplying PLL in which a single reference clock is phase compared to two local clocks derived using different integer divisor ratios, with interpolation between the two phase error results dynamically chosen to cancel the phase quantization error.

Phase Detector with Interpolator

As communication channel data rates increase, it becomes increasingly difficult to maintain acceptable PLL lock range and accuracy, as inherent and parasitic circuit node capacitances introduce circuit delays and constrain the effective loop response bandwidth. An embodiment providing improved response characteristics suitable for such high speed operation is illustrated in FIG. 5. As one familiar with the art will observe, this is a CMOS design providing symmetrical operation for both positive and negative output excursions, integrating elements of both phase interpolator and phase detector designs. This tight integration results in reduced node capacitances, facilitating the desirable high speed operation, and the balanced differential structure simplifies the control of charge and discharge currents.

As with conventional designs, the PLL VCO (or a clock divider driven by said VCO) provides the local oscillator inputs to phase interpolator elements 510 and 515, which together set the effective local clock phase. Four local oscillator phases with 90 degree offset are shown i.e. equivalent to two phases in quadrature relationship and their complimentary signals and thus identified as +I, +Q, and −I, −Q, permitting a full 360 degree or “four quadrant” phase adjustment. Other embodiments may utilize as few as two local oscillator phases, may use oscillator phases having other than 90 degree phase differences, or may select clock phases from an input set of more than four; as one non-limiting example, choosing at least two clock phases to be interpolated between from an input set of eight clock phases.

In a first embodiment, phase interpolator element 510 includes four mixing elements, each mixing element comprising a differential transistor pair and a controlled current source, with a common differential output driven by the four mixing elements in parallel. Thus, configuration of current source IA(i) controls the amount of local oscillator phase +I presented to the common output ckp; similarly, current source IA(−i) controls the amount of complimentary output phase −I in the output, IA(q) controls the amount of +Q, and IA(−q) controls the amount of −Q. It will be readily apparent to one familiar with the art that configuration of the four current sources can produce an output clock at Ckp having any desired phase relationship to the PLL local clock input.

Similarly, phase interpolator element 515 current sources IB(i), IB(−i), IB(q), and IB(−q) may be configured to obtain an output clock at Ckn having any desired phase relationship to the PLL local clock input. Typically, CkPLLp and CkPLLn will be configured to have complimentary relationships so as to provide phase detector 520 with balanced and complimentary positive- and negative-going current amplitudes. However, configuration with non-complimentary IA and IB values may be performed to obtain particular results. As one example offered without limitation, an embodiment separately adjusting IA and IB values might obtain higher resolution phase adjustment, compared to an embodiment maintaining perfectly complimentary IA and IB values.

The second input to the Phase Detector 520 is external reference clock CkRef+/CkRef−, producing the phase error output currents VCOctl+/VCOctl−. In one advanced embodiment, the two external reference clocks are of opposing polarity but not necessarily complementary phase, thus the positive polarity comparison and negative polarity comparison represent different phase comparisons. Such an advanced embodiment may be combined with non-complimentary IA and IB bias configurations, providing independent adjustment of local clock phase during those different phase comparisons. That is, in one embodiment, the CkRef input at the top of PD 520 is a first phase selected from the reference clock phases available in the circuit, and the IA currents are adjusted to provide a corresponding interpolated phase offset from the first selected phase, and the CkRef input at the bottom of PD 520 is a second phase selected from the reference clock phases available in the circuit, and the IB currents are adjusted to provide a corresponding interpolated phase offset from the second selected phase, wherein the amount of the relative phase offsets are the same.

Configuration of phase interpolator current source values may be performed by external control logic, including without limitation, a hardware configuration register, control processor output register, and hardware CDR adjustment logic.

Alternative Phase Detector Embodiments

Phase Detector 520 in the embodiment of FIG. 5 is shown as an XOR-style device as in FIG. 4A, mixing local clock CkPLL and external reference clock CkRef to produce phase error output VCOctl. In the alternative embodiment of FIG. 12A, a folded phase detector is used at 1220, driven by currents produced by the combination of phase interpolator 510 and current sink Ifix2, and the combination of phase interpolator 520 and current source Ifix1. The folded phase detector embodiment shown in FIG. 12A is described in further detail below. As with the previously-described embodiment, current sources IA(i), IA(−i), IA(q), and IA(−q) are configured to produce the desired interpolation of PLL clocks i, −i, q, and −q in interpolator outputs CkPLLp and CkPLLp, while current sources IB(i), IB(−i), IB(q), and IB(−q) are configured to produce the desired interpolation of PLL clocks i, −i, q, and −q in interpolator outputs CkPLLn and CkPLLn. Phase comparator 1220 is also driven by received reference clocks CkRef+and CkRef−, producing phase comparison results Phase Error (+) and Phase Error (−). In some embodiments, the circuit node labeled Circuit Balance Feedback may be monitored to determine the relative DC component of the interpolated clock signals, which may then be modified by adjustment of the configured current source values in 510 and 515. In some embodiments, each current source IA and IB receives seven control bits. It should be noted that embodiments are not limited to receiving seven control bits, and that any number of control bits may be implemented according to design constraints for PI resolution, for example. In some embodiments, current sources IA and IB are equal (e.g., IA=IB for +/−i, q). In such embodiments, the PIs 510 and 515 have 7 bits of resolution. In alternative embodiments, additional resolution may be implemented by introducing a shift in IB with respect to IA, or vice versa. In an exemplary embodiment, IA=IB+8, where 8 is a decimal shift added to the control bits of each current source IA to obtain the control bits of each current source IB. In such embodiments, the P-side PI 510 and N-side PI 515 are looking into two different VCO phases, and the phase detector collects information from different phases of the VCO. Since the PIs 510 and 515 combine information from different phases of VCO, the PLL has more detailed information about phases of PLL and the bandwidth of the PLL is higher than a conventional PLL.

Embodiments for which IA=IB+shift are a special case of a matrix phase comparator in which there are two phase comparators. The first phase comparator (NMOS-side XOR) compares the phase of reference with one set of VCO feedback phases, and a second phase comparator (PMOS-side XOR) that compares the reference clock phase with a second set of VCO feedback phases. Unlike the [Nandwana] phase comparator, the sets of VCO feedback phases here are of the same frequency, differing only in phase, and the current source values chosen to interpolate between the phase comparison results will typically be static rather than dynamically chosen on a cycle-by-cycle basis. Matrix phase comparator embodiments are described in further detail below. Therefore, in some embodiments, a PMOS+NMOS interpolator may be treated as two independent PIs, while in [Nandwana], there is only one PI. Further, if a meaningful difference to the weighs of PMOS-side and NMOS-side, then a small matrix PLL may be constructed that has extended bandwidth. In at least one embodiments, there is a 20% gain difference between the two sides, in which BW may enhance by the same factor.

In some embodiments, a folded structure as shown in FIG. 12A may be used. FIG. 12A is similar to the embodiment shown in FIG. 5, however the phase detector 520 is replaced with a folded phase detector 1220. As shown, folded phase detector 1220 includes current sources Ifix1 and Ifix2, which may be configured to provide more voltage headroom to the PMOS PI current sources IA and the NMOS PI current sources IB. Further, phase detector 1220 includes a pair of transistor branches connected to CkPLLp and CkPLLn. For purposes of illustration, consider PI 510 and 515 only having IA(i) and IB(i) turned on respectively, representing phase ph0000 from the VCO. In the case where CkRef is offset 90 degrees from ph0000, the folded phase detector 1220 will be in lock condition. As shown in FIG. 13A, during the first 180 degrees (1) of a period, for a first 90 degrees (2), current Ip is charged to the (−) terminal of the Phase Error signal through transistor 1206 using PMOS PI 510. At the same time, current In is discharged from the (−) terminal of the Phase Error signal through transistor 1208 using NMOS PI 515. Similarly, during the second 90 degrees (3), current Ip is charged from the (+) terminal of the Phase error signal through transistor 1202, while current In is discharged from the (+) terminal through transistor 1204. As shown, Ifix2 will sink a fixed amount of current being provided from PMOS PI 510, while Ifix1 sources some current to NMOS PI 515 to prevent the current sources in the NMOS PI from sinking too much current from the Phase Error signal. Such a technique provides a reverse clipping effect. One of skill in the art may notice that equally adjusting the Ifix current magnitudes may have an effect on the range of the Phase Error signal. In some embodiments, increasing the Ifix magnitudes will lower the magnitude range of the Phase Error signal, while decreasing the Ifix magnitudes will increase the magnitude range of the Phase Error signal. This relationship can be found in FIG. 13B.

FIG. 13B is a timing diagram illustrating the reverse clipping feature described above. FIG. 13B depicts the magnitude of current Ip in the first 180 degrees (1) for two values of Ifix2: A and B, where A>B. As shown, the magnitude of Ip is less in the case of Ifix2=A. When Ifix2=B, the magnitude range of Ip is relatively higher. One of skill in the art would notice a similar effect occurs in the case of In being discharged from the folded phase detector 1220.

In some embodiments, the second 180 degrees (4) may be used to provide circuit balance feedback, as shown in FIG. 12A. During the circuit balance feedback phase (4), current may be charged via the PMOS PI 510 while current is discharged via the NMOS PI 515. If there is an imbalance of charge/discharge currents, a non-zero circuit balance feedback signal may indicate this imbalance, which may occur due to transistor mismatches, for example. The circuit balance feedback signal may then be used to adjust either Ifix1 or Ifix2 to balance the charge/discharge currents so that the balance feedback signal is zero. In some embodiments, the voltages of the charge-pump circuit may be monitored, and if equal, the circuit is properly balanced, i.e., Ip=In. A simplified schematic of the phase comparator circuit of FIG. 12A is shown in FIG. 12B.

The phase detector of [Tajalli V] may alternatively be used at 520 or 1220, providing equivalent phase detection with enhanced signal headroom in embodiments utilizing low power supply voltages. Other phase detectors, including all variations shown in FIGS. 4A, 4B, and 4C, may also be substituted at 520 in that embodiment.

As one example of such alternative embodiment, the State Machine Phase/Frequency Detector of FIG. 4B may be combined with the Phase Interpolator design of FIG. 5.

FIG. 6A shows a schematic of one embodiment of a conventional CIVIL clocked latch, composed of a clocked feedback latch outputting results Q and Q the state of which is initialized by clocked differential inputs D and D. FIG. 6B shows the same circuit in which the clock source phase is modified by phase interpolator 615, operation of which is as previously described for FIG. 5.

Substituting the clocked latch circuit of FIG. 6B into each D flip-flop instance of FIG. 4B produces the alternative embodiment shown in FIGS. 7A and 7B. D flip-flop 710 is clocked by the received clock CkRef, which is passed through phase interpolator 715. As an example and for purposes of explanation, without a configured phase offset (or a desired offset of 0 degrees), current source IA would be set to “mix” input CkRef at 100% proportion, and the other three current sources set to zero current. D flip-flop 720 is clocked by local clock CkPLL, which is obtained by configuration of phase interpolator 725 current sources IB(i), IB(−i), IB(q), and IB(−q), which in turn controls the relative proportions and polarities of I and Q clocks being combined. In one embodiment, I is obtained from ph000, −I from ph180, Q from ph090, and −Q from ph270, as seen in FIG. 3. A simple CIVIL OR gate 730 drives the reset function for flip-flops 710 and 720.

It should be noted that in this one embodiment the majority of phase interpolator 715 is functionally disabled and retained only to preserve the same parasitic load characteristics as are presented by active phase interpolator 725, to maximize circuit symmetry and maintain balanced loading characteristics to minimize secondary effects such as detection bias and drift.

Integrated Phase Detector, Interpolation, and Charge Pump

As previously described, PLL phase detector outputs are typically used to drive a charge pump circuit, the output of which is an analog error signal used to control the VCO. The described improvement from reduced capacitance and resulting higher circuit speed in integrating the PLL phase detector and clock adjustment phase interpolator may be further extended by also integrating elements of the charge pump in the same manner.

In this combined embodiment, the charge pump control signals UPp, UPn, DOWNp, and DOWNn provided by the embodiment shown in FIGS. 7A and 7B directly control the charge pump embodiment of FIG. 8 to produce output IOUT. Current source ICPC and voltage reference VREF may be configured to scale and adjust the IOUT range. One familiar with the art will note the significant symmetry in the circuit of FIG. 8, allowing accurate tracking between generation of VREPLICA and IOUT signals.

FIG. 8 is a schematic of a charge pump circuit with improved charge/discharge current balancing, in accordance with some embodiments. The circuit 800 includes two parallel charge pumps 802, 804: the two differential pairs within charge pump 804 generate an output current representing a phase error in response to the up and down pulses, and the two differential pairs of charge pump 802 are used to set the discharge current to be equal to the charge current as described below. Specifically, the current source ICPC sets a charging current level by providing a corresponding bias voltage VBP through a current mirroring circuit to drive the top current sources 806, 808 of the two charge pumps so as to also provide ICPC to each charge pump 802, 804. When UPn goes low and turns on FET 810, the node 812 is charged (capacitive element 814 is either a discrete cap or a parasitic cap) by the charging current ICPC provided by FETs 806, 810. In a balanced condition (i.e., in the absence of a phase error), the amount of current that is then discharged during a high DOWNp signal through the bottom FET 816 should bring the node 812 back to the VREF value. If the discharge current is too low and the voltage VREPLICA increases above VREF, then the amplifier 820 will increase the bias voltage VBN to the discharge-current FET 818 to increase the amount of discharge current so that it equals the charge current ICPC and the voltage VREPLICA at node 812 is brought back to VREF. On the other hand, if the discharge current set by VBN on FET 818 is too high, the VREPLICA voltage drops too low, and the amplifier 820 responsively reduces the bias voltage VBN on discharge-FET 818 to bring the charge pump currents into equilibrium.

Second order PLLs (called also charge pump PLLs) have been widely used to implement low noise and high performance synthesizers, clock generators, and clock and data recovery systems. A phase detector (PD), or a phase-frequency detector (PFD) produces a signal proportional to the phase difference between the reference clock (CkRef) and the feedback clock (CkPLL). The resulting error is integrated by charge-pump circuit (CPC) and loop filter (LF) to produce the proper control voltage for the voltage (or sometimes current) controlled oscillator (VCO). An exemplary LF is an RC circuit as shown in FIG. 24. Many modern integrated VCOs are based on differential topology that can provide two complementary outputs. A differential architecture provides more resistivity against supply and substrate noise. LC tank based VCOs and ring oscillators are two main categories of controlled oscillators that have been very widely used in high speed communication systems. Both topologies can be configured to provide two or more output phases, which is essential for multiphase systems and facilitates the enhancements described below.

Other embodiments may be obtained by equivalent combination of phase comparator, phase interpolator, and charge pump elements.

Oversampling of Input Reference Signal

The asymmetric use of the phase interpolators in, as one example, FIGS. 7A and 7B, stems from the different nature of the local clock and reference clock sources. The former is obtained from a multiphase clock source (e.g. an oscillator or divider) inherently capable of providing the multiphase inputs utilized by a phase interpolation element. The latter is generally single phased, obtained from (typically) one received clock source.

In the known art, [Tan] described a combined DLL/PLL structure, in which the voltage controlled delay line incorporated in the PLL VCO is duplicated as an input delay line acting on the reference clock input, and controlled by a single feedback error signal. [Ng] and [Chang] also describe use of a front-end DLL to serve as a frequency multiplier to facilitate generation of very high frequency clocks.

However, if such a controlled delay line is tapped, and so configured that the differential delay between taps is proportional to the time between received clock edges, a received clock passing through such a delay line produces a resulting set of outputs which take on some of the characteristics of a multiphase clock. As one example offered without limitation, the equal-interval outputs of a four tap delay line having an overall delay comparable to the reference clock period will provide outputs having similar characteristic to quadrature phased clock signals. Continuing this example, if each such output is phase compared to an appropriately-selected local clock phase, a series of phase error results will be produced which may be combined to produce a more accurate aggregate clock error signal for the PLL VCO. The delayed versions of the receive clock represent additional opportunities for phase comparison with a clock derived from the VCO, thus providing a higher update rate for the controlled loop, and thus improved PLL loop bandwidth leading to reduced jitter and better noise immunity. That is, using this technique, the update rate of the loop will be increased, which in turn enables the circuit to track and correct the effects of noise and jitter at higher frequencies.

For the delayed phase comparisons to provide meaningful information to the PLL, the delay intervals provided by the delay line must be coordinated with the period between local clock phases, with such controls giving the delay element many of the aspects of a Delay-Locked Loop (DLL.) As seen in the block diagram of FIG. 9, the external clock reference input to the previous PLL embodiment 300 is provided by DLL 910. The received clock signal R5 is presented to tapped delay line 916, producing a series of received clock phases 918. The DLL control loop is provided by phase comparator 912 comparing the received clock with a delayed clock, producing an error value that is Low Pass Filtered 915, producing a Delay Adjust signal controlling the delay line timing.

Within PLL 300, the previous simple phase comparison (320 of FIG. 3) is now performed by multi-phase comparison 920. In one embodiment, XOR gates compare the phase of each received reference clock phase on the N lines (e.g., N=2, 4, 8, etc., and possibly including odd integers as well to obtain other phases such as 60, 120, 180, 240, 300) 918 with a different clock phase from the N lines 965 from phase interpolator 360. Each XOR gate output is converted to an analog signal value, all such analog signal values being summed to produce a composite analog Error result controlling ring oscillator 340, as previously described. In a further embodiment, summation 935 is performed by a weighted summation node comparable to the previously-described MIC mixer, the different selected weights of said summation allowing further control of PLL static and dynamic operational characteristics. Alternatively, each XOR output can be used to drive a separate transistor circuit for injecting or removing charge from a capacitive element to achieve the summation. In a further embodiment, each XOR phase comparator may include a plurality of AND operations implemented as transistor branches, each AND operation configured to provide a current output to a common summation node, the magnitude of each current being independently configurable so as to provide a weighting function to each AND operation. In addition, the PLL 300 of FIG. 9 may be configured to provide a desired phase offset, where the interpolated phases each have the same offset relative the tap delay line signal to which it will be XOR compared.

In some system environments, the described multi-phase reference clock may be directly available from the receiver, as one example where the communications protocol incorporates multiple clock signals.

The additional feedback information provided by the multiple comparison operations may also be obtained without the previously-described DLL front end. FIG. 10 shows an embodiment in which the single received reference signal 1018 enters multi-phase comparator 920 in which the single received reference signal is compared to each of two or more local clock phases 965. As in the previous example, this multiple phase comparison is distinct from that of [Nandwana] in that all of the local clock phases used for comparison are of the same frequency, differing only in phase. In one embodiment, XOR gates compare the phase of the single received reference clock phase 918 with a different clock phase 965 from phase interpolator 360. Each XOR gate output is converted to an analog signal value, all such analog signal values being summed to produce a composite analog Error result controlling ring oscillator 340, as previously described. In a further embodiment, summation 935 is performed by a weighted summation node comparable to the previously described MIC mixer, the different selected weights of said summation allowing further control of PLL static and dynamic operational characteristics. In another embodiment, each XOR phase comparator provides a current output to a common summation node, the magnitude of each current being configurable so as to provide a weighting function. In particular, such weight adjustments may be used to produce additional closed-loop poles and/or zeroes in the PLL time domain transfer function, providing additional control of loop stability.

FIG. 14A is a timing diagram of a reference signal CKREF being compared with four phases of the VCO (feedback from the PLL):

-   XOR(CKREF, VCO′000) -   XOR(CKREF, VCO′045) -   XOR(CKREF, VCO′090) -   XOR(CKREF, VCO′135)

As shown in FIG. 14A, it is assumed all weights are equal, however this is purely for illustrative purposes, and should not be considered limiting in any way. FIG. 14A further includes a summation of the four XOR outputs. As can be seen, in lock condition, the integral of the bottom waveform is zero, and the PLL will lock properly. For convenience, FIG. 14B has been included to illustrate a conventional XOR based phase detectors in which the reference is compared to only one VCO phase. In lock position, the reference and VCO are 90-degree phase shifted, and the output of XOR is a rectangular waveform with an average value equal to zero. One may observe how the two waveforms (simple XOR in the FIG. 14B and array-XOR in FIG. 14A) differ from each other; however in both cases the average value for a given period is zero, and the PLL locks. In embodiments utilizing an array phase detector, a larger number of transitions occur with respect to a single XOR phase detector. As each transition carries information about an edge, a larger number of transitions means that phase comparator has been able to collect more information from VCO and CKREF.

It should be noted that in array-XOR embodiments, some comparisons might be done using XNORs. As such, an XOR or XNOR for different phase comparisons may be selected carefully to ensure system stability.

In at least one embodiment, the weights of said summation are configured such that they decline in proportion to the timing difference of the comparison clock phase relative to the PLL “normal lock” phase. As one example offered without limitation, if ph090 is the normal lock phase of the PLL, the comparison of ph090 and the received reference signal is weighted 1; comparisons of ph045 and ph135 (e.g. a half tap offset from the normal lock phase) are weighted ½; comparison of the received reference signal and ph000 and 180 (one tap offset from the normal lock phase) are weighed ¼; etc. These various weighted comparison results are then summed to produce a composite signal which when low pass filtered 330, is the Error value controlling PLL VCO 340.

In one embodiment utilizing multiple phase comparators, the deterministic jitter produced by the multiple phase comparisons was seen to occur at a 12.5 GHz rate with equal phase detector weights. Even though the amount of jitter was very small and the jitter rate was well above the loop filter cutoff frequency, the deterministic jitter was significantly reduced with the described weight adjustments, in which weight magnitudes decline in proportion to their offset distance from the primary reference signal sample. In some embodiments, different weighted values are used in a comparator circuit to construct a discrete time domain filter. This property can be used to simplify the design of analog filter 330. For example, with proper weighting values one might construct a discrete time domain zero in the transfer function that provides conditions to make the loop robust.

As with previously described examples, other embodiments may be obtained by equivalent combination of phase comparator, phase interpolator, and charge pump elements.

Matrix Phase Comparisons

In some embodiments, BW of a PLL is limited by the update rate of the loop, which is determined by the frequency of the reference clock. Certainly, using all the available sources of information in system can substantially enhance the efficiency of the correction loop. For example, every phase of the VCO provides a single sample of the oscillator phase during each cycle of the reference clock period, while looking into all the phases of the VCO can provide more detailed information in the time span of Tref shown in FIG. 14B. In conventional PLLs, only one of the VCO phases is fed back into the phase detector. Hence, the phase detector has only part of the available information regarding instantaneous phase of oscillator. The following embodiments utilize different ways of improving loop update rate using a two dimensional phase comparator.

The multi-phase comparison of multiple phases derived from a received reference signal and multiple phases derived from the local PLL clock may be generalized into a matrix phase comparator, one embodiment of which is shown in FIG. 11B, with one embodiment of each individual phase comparator in the matrix shown in FIG. 11A. For descriptive purposes, XOR partial phase comparators arranged in a four by four matrix are illustrated, with no limitation implied by those illustrative choices. Embodiments may be organized into rectangular, square, or sparse matrices of any dimensions M and N, with elements of the matrix being composed of any phase comparator described herein and optionally any weighting factor computation described herein. As the local clock phases repeat cyclically, the leftmost and rightmost columns of the matrix should be considered as being logically adjacent in their local clock phase relationship. This may be observed in FIG. 11B. Suppose in FIG. 11B, CKPLL0=0°, CKPLL1=90°, CKPLL2=180°, and CKPLL3=270°. It should be noted that these numbers are being used purely for illustration. It would thus follow that a fifth local oscillator clock CKPLL4 would equal 360, which would of course be the same phase as CKPLL0. Thus, the left-most and right-most columns should be considered adjacent in accordance with embodiments described above. As used herein, a sparse matrix is any embodiment in which at least one of the described element weights is zero. In some embodiments, one or more number of reference clock phases can be compared to one or more than one phases of the feedback clock. Every extra feedback clock phase (CkPLL_(N)) provides more detailed information about the phase noise of VCO in time domain. Hence, such a phase comparator may more often provide correction signals to the loop filter. In other words, a multi-phase feedback system enables the loop to increase its update rate and correct for phase deviation of VCO at higher rates. Similarly, if there are more number of reference clock phases available, higher resolution phase comparison can be made, and correspondingly correction signals may be applied more often in time. If there is only one reference phase, still a controlled chain of delay line (CDL) can be employed to replicate the reference clock. The bandwidth of control loop for such a delay line needs to be either very high, or very low in order to guarantee that all the phases of the replicated clock phases carry similar jitter characteristics within the frequency range of interest. A generalized two-dimensional phase comparator is depicted in FIG. 11B, in which every phase of the feedback signal can be compared with every phase of the reference clock.

In a full matrix comparison, each of M phases derived from the received reference signal is separately phase compared with each of the N phases derived from the local PLL clock. Each resulting phase error signal is weighted by a configured or predetermined amount, with all (M*N) weighted results summed to produce an aggregate error result. An example of one partial phase comparator is shown in FIG. 11A as 1110, composed of XOR phase detector 1112 feeding to result weighting factor 1118. As shown, each partial phase comparator 1110 receiving CKRef(m) and CkPLL(n) may have a corresponding weighting factor W(m,n) for 0≤m<M−1 and 0≤n<N−1. An embodiment of the complete matrix phase comparator 920 in FIG. 11B is composed of M*N instances of 1110, each accepting one of the M reference phases, herein identified as CkRef0, CkRef1, CkRef2, CkRef3, and one of the N local phase inputs herein identified as CkPLL0, CkPLL1, CkPLL2, CkPLL3, and producing a weighted result e.g. multiple results 1131, 1132, 1133, 1134 as input to summation 935, which produces an aggregate result 1145.

One familiar with the art will observe that the previously-described multi-phase comparator 920 of FIG. 9 is equivalent to a partially-populated instance of the present matrix comparator, i.e. having comparators instantiated across a diagonal of the matrix. Functionally, an identical result may be obtained from a full matrix by setting the weights along such a diagonal to a nonzero value, and all other comparator weights to zero. It thus follows that other described behaviors including simulation of phase offsets, introduction of loop time domain zeroes, etc. may be similarly be obtained by selective configuration of matrix weighting factors. In such embodiments, each reference clock phase will be compared with its corresponding feedback clock phase, i.e. CkRef,m versus CkPLLn. The dynamics of such a diagonal comparator are similar to a conventional PLL (CPLL) except that the update rate is N times faster. Higher loop update rate will allow a diagonal PLL to track input jitter and correct jitter of VCO up to higher frequencies. In summary, signal (reference) transfer characteristics, (STF), and VCO noise (jitter) transfer characteristics, (NTF or JTF), of such a system will be N times wider compared with the conventional PLLs.

In at least one embodiment, the matrix comparator of FIG. 11B may be simplified such that different phases of feedback signal (CkPLL) are compared against only one reference clock phase. Such embodiments are especially interesting as generally there is only one single reference clock phase available. In such an implementation W(m,n) are zero unless m=0.

Assuming a four phase feedback, each phase comparator produces a correction signal which eventually will be accumulated onto the capacitor of the loop filter. While under lock condition the integral of the correction signal illustrated in the bottom waveform of FIG. 14A is zero, this signal has its main harmonic at 2f_(ref). Any jitter on the reference clock or feedback divider phase will cause some deviations at different edges of the bottom waveform shown in FIG. 14A. The bottom waveform in FIG. 14A shows the correction signal injected into the loop filter. Comparing the waveform depicted in the bottom waveform of FIG. 14A with the waveform of a simple XOR phase comparator shown in FIG. 15, it is evident that the number of transitions happening at the output of comparator during a single reference clock period has been increased by more than a factor of two. While in both waveforms there are two jumps created by the rising and falling edges of CkRef the number of transitions due to feedback signal has been increased from two in FIG. 15 to eight in FIG. 14A (also shown in FIG. 16). Based on this, the feedback transfer function in a multi-phase diagonal PD will be different form a conventional PD. Due to more number of samples provided by the feedback path in this case, the noise of VCO can be corrected more often, and hence over a wider frequency bandwidth.

A multi-phase or array phase comparator as described above opens new doors to design low noise and wide-band PLLs. Described below are different examples of improving performance of PLLs using array phase comparators.

In some embodiments, array phase comparators provide double edge phase comparison: Using double edges (rising and falling edges) of the reference clock provides the possibility to make two separate phase corrections at every cycle. Hence, the BW of PLL can be potentially increased by a factor of two. In case the duty-cycle of the input reference is not 50%, it will create some ripples at 2·Fref and can increase deterministic jitter (DJ) of the oscillator. Indeed, as the ripple frequency is fairly high, with proper loop filter design it will be possible to reject major part of this DJ.

In some embodiments, array phase comparators provide Inter-VCO-phase comparison. To detect and correct duty-cycle and quadrature mismatch errors (QME), generally designers make comparison between different phases of a VCO. For such correction, the error signal resulted from inter-VCO-phase comparison is heavily filtered and a very low frequency correction signal is applied in a proper point of the system in order to correct for duty-cycle distortion or quadrature mismatch. Embodiments described above may be expanded to detect and correct random jitter of VCO.

Some embodiments of the two-dimensional discrete-time phase comparator provide the possibility to implement a discrete-time filter in front of PLL. This opportunity can be used to implement special transfer functions in order to improve the performance of system. For example, by proper choice of digital filter coefficients (e.g., the weights described above), one may detect and suppress phase noise at special frequencies in order to improve tolerance of system against supply or substrate noise. Some embodiments provide Phase Interpolation: There are different approaches for rotating phase of oscillator in a PLL and adjusting the exact timing of recovered clock based on system requirements. In some embodiments, a diagonal comparator array structure provides the possibility to rotate a phase of the oscillator with respect to the phase of reference clock.

Suppose for a given reference clock CkRefm and a given local oscillator phase CkPLLn, W(m,n)=a for (m−n)=0, W(m,n)=b for |m−n|=1, and zero for the rest of m and n values (noting that n must be considered modulo the number of local oscillator phases, because of its previously-described cyclic nature. Assuming that a+b=c, and c has a fixed value. In this case, by changing a, and b=c−a, it is possible to rotate the phase of VCO. If lock points corresponding to [a,b]=[c,0] and [a,b]=[0,c] are Ta and Tb , respectively, then as depicted in FIG. 17, by changing a (and inversely changing b=c−a) the lock point of oscillator can be adjusted between Ta and Tb.

The same argument is valid for any other combination of the two consecutive sets of |m−n|=k and |m−n|=k+1, where k is an integer number smaller than the size of phase comparator matrices. Suppose a and b are the weights for two consecutive sets of diagonal comparators k and k+1. If a and b are controlled digitally with two independent sets of N_(b) bits, then the resulted phase interpolator can exhibit N_(b)+1 bits of resolution. Illustrated in FIG. 18, it can be observed that by proper jogging between n(a) and n(b) (control bits corresponding to a and b, respectively), it is possible to add one extra phase point between every two phase steps of the original phase interpolator.

An example of diagonal interpolation is given below, where the main diagonal has a weight a assigned to each element and where an adjacent diagonal has a weight b assigned to each element:

$\begin{bmatrix} a & b & 0 & 0 \\ 0 & a & b & 0 \\ 0 & 0 & a & b \\ b & 0 & 0 & a \end{bmatrix}\quad$

Note that due to the cyclical nature of adjacent columns, a weight on the element of b is included at the bottom-left most element of the matrix. Thus, it always remains that an equal number of elements will contain a weight of a and b, providing a linear and symmetric phase relationship, as shown in FIG. 17. Another example of two adjacent diagonals is given below showing the cyclic nature:

$\begin{bmatrix} 0 & a & b & 0 \\ 0 & 0 & a & b \\ b & 0 & 0 & a \\ a & b & 0 & 0 \end{bmatrix}\quad$

In an alternative embodiment, interpolation can be performed between two adjacent rows or two adjacent columns of a matrix. The concept is very similar to the diagonal embodiment above, in that a first column may have a first fixed phase offset, while a second column has a second fixed phase offset. A weight a may be assigned to each element in the first column and a weight b may be assigned to each element in the second column, where a+b=c, as described above. Also, it should be noted that the left-most columns and right-most columns should be considered adjacent due to the cyclical nature of the local oscillator clocks. Thus, interpolating the weights a and b in each column will provide an intermediate phase in between the first and second fixed phases.

Phase Comparator Architecture

The flexibility and architectural simplicity of a matrix comparator may be offset by the potential complexity of its embodiment, in both the number of comparators used to fill an array of M×N elements, and in the implementation of the required weighting or scaling factor for each of those elements.

As a non-limiting example, a PLL utilizing a matrix comparison of each of eight VCO clock phases against a single reference clock input includes eight comparator elements, each associated with a configurable or adjustable scaling factor, and one summation node to produce a combined error result. In one such embodiment shown in FIG. 19A, each partial phase comparator 1110 incorporates a multiplying DAC structure to implement the weighting factor as in 1118. In some embodiments, the weighting signal selectively enables one or more switching elements, which may include a transistor switch and a current source, for example. Thus the more switching elements are enabled, a higher weight will be applied to the partial phase-error signal.

As the distributed capacitance of the DAC structure in the signal path may unreasonably degrade signal integrity, an alternative embodiment moves the DAC out of the signal path. In this second embodiment, a DAC element 1116 is used to scale or adjust supply voltages to a digital buffer element 1115 as in FIG. 19B, resulting in a scaled or weighted signal output.

Circuit applications requiring low power operation may be incompatible with resistive DAC usage, thus a further embodiment utilizes switched capacitor technology. In this third embodiment of FIG. 19C, the scaled or adjusted supply voltages for buffer 1115 are produced dynamically 1117, by dumping measured amounts of charge from one or more source capacitances C1 into the buffering device's power rail capacitance C2. In this illustrative example switch SW1 is shown performing this charge transfer; switching transistors would be used in a practical embodiment, as well understood in the art, and either or both of C1 and C2 may be composed of discrete as well as parasitic or distributed capacitances.

An alternative fourth embodiment shown as FIG. 19D does not attempt to adjust or modify the signal output of a single phase comparator, but instead utilizes a parallel set of phase comparators 1113 which collectively produce an aggregate output signal. In such an embodiment, the output drive capability of an individual phase comparator 1113 is constrained, e.g. by the transistor dimensions of its output driver. As one example, an aggregate output may be produced by passive summation of the individual phase comparator current outputs, and the overall output amplitude weighted or controlled by enabling or disabling phase comparators within the set, either individually (as one example, controlled by a thermometer code,) or in groups (as another example, controlled by a binary weighted code.)

During high-speed simulation of that fourth embodiment, transient output variations were observed within the gate propagation time of the XOR phase detector element, that is, at a finer granularity than the overall behavior of the XOR gate as a whole. It was noted that an exclusive-OR function may be decomposed into distinct NOT-AND-OR logic elements as described by the well-known Boolean equation (x·y)+(x·y), and the observed behavior suggested that variations among the different current paths within the gate implementation were producing the observed variations. This lead to the embodiment of FIG. 20, where the four signal paths 2010, 2020, 2030, 2040 are composed of series transistors forming transistor branches configured to respectively compute the logical cases x·y, x·y, x·y, x·y, each path further includes an adjustable or configurable impedance which may be a resistor, a current source/sink, or in some embodiments may be implemented as a scaling of transistor geometry to constrain current flow in that signal path. A timing diagram illustrating the output lout resulting from inputs X and Y is shown in FIG. 21.

Adjustment of the four signal path impedances by introducing path resistance, scaling transistor dimensions, or directly adjusting current flows may be used to control output amplitude within the overall response of the XOR gate as a whole, thus producing the desired weighing function. As one example, consider each signal path impedance in FIG. 20 as being composed of four parallel resistors each controlled by a transistor switch. In such a case, a portion of a weighting signal which may be a four-bit thermometer code t₀-t₃ (as one example offered without limitation) can enable four distinct levels of current flow within signal path 1920, which may be seen in FIG. 21 to adjust one segment 2110 of the overall output lout. Continuing this example, t₄-t₇ adjusts segment 2120, t₈-t₁₁ adjusts segment 2130, and t₁₂-t₁₅ adjusts segment 2140. Although in some embodiments the weights of each of the four segments will be adjusted identically, this is not required. As examples, t₀-t₃ and t₄-t₇ may be configured to different values than t₈-t₁₁ and t₁₂-t₁₅ to provide increased overall adjustment granularity as previously described relative to FIG. 5. As another example, t₀-t₃ and t₈-t₁₁ may be configured to different values than t₄-t₇ and t₁₂-t₁₅ to intentionally introduce a DC offset at the output.

Independently adjusting the four segments of each XOR operation may also facilitate the previously-described matrix comparator operations, including interpolation. As one example, two XOR comparators as in FIG. 20 with outputs connected to a common summation node may be used to compare a reference clock with two local clock phases, as previously described. If interpolation control values a, b, c, d represent the weighting signals applied to the first XOR segments (t₀-t₃, t₄-t₇, t₈-t₁₁, t₁₂-t₁₅), and 1-a, 1-b, 1-c, 1-d represent the weighting signals applied to the second XOR segments, the common summation node result corresponds to an interpolation between the first and second local clock phase comparisons, with the interpolation control values permitting adjustment of the effective clock phase.

Alternatively, scaled transistors or explicit current source/sink circuits may be used instead of resistors to control current flow; as in the resistive example, different numbers of enabled and disabled parallel current paths may be used to adjust the overall output Iout. the portions of the weighting signal above collectively form the weighting signal t₀-t₁₅ for the corresponding partial phase comparator.

The adjustable or configurable elements described herein may be combined with or equivalently be substituted by comparable known art elements, including without limitation R-2R ladder structures controlled by transistor switches, resistive chain structures controlled by transistor switches, equal-value or binary-weighted resistors configured in series- or parallel-connected combination by transistor switches, and fixed and/or configurable current sources and current sinks.

Specific values and quantities described in examples are provided to facilitate explanation, without implying limitation.

Simulations

The steady state response of two different types of phase comparators are shown in FIG. 22. In these simulations XOR based phase comparator cells have been utilized. The gain of a diagonal phase comparator versus input phase difference is very similar to a simple XOR phase comparator. The main difference between the two phase comparators is that error signal produced by a diagonal phase comparator is distributed in time. The row phase comparator, however, exhibits a different response. As can be seen, depending on which row of the matrix comparator has been selected, the response exhibits a different shift in time. FIG. 22 shows the response for two different cases.

An MCPLL (matrix phase comparator based CPLL) has been designed in a conventional 28-nm CMOS technology. A current steering architecture with K_(VCO)≈20 GHz/V is selected for the VCO. A loop filter as shown in FIG. 24 is chosen to be R1=100 Ω in series to C1=20 pF with ICPC=100 μA. The reference clock frequency is 6.25 GHz with N_(div)=1. The XOR CPLL is based on 2R×2F architecture (comparing 2 reference phases with 2 feedback phases at every reference clock cycle). For the implemented MCPLL, a 2R×4F architecture has been chosen.

FIG. 23 shows the transfer characteristics of the two PLLs simulated in transistor level. As shown, the input signal is sampled at 2fref, hence the system Nyquist rate is fref. Because of that, as depicted in FIG. 22, both PLLs have a null at fref=6.25 GHz. The parasitic pole of the VCO in addition to the parasitic pole at the output of CPC cause 1.2 and 2.4 dB peaking in CPLL and MPLL, respectively, for the two PLLs. It can be seen also that the MCPLL transfer characteristic stays closer to 0 dB between 100 MHz to 1.2 GHz. The cutoff frequency for the two PLLs are 2.48 (=40% of fref) and 5.02 GHz (=80% of fref), respectively. Therefore, it can provide better jitter tracking. Moreover, MCPLL generates 55 fs of rms jitter, while jitter generation for CPLL is 79 fs rms. Due to higher BW, the phase noise of VCO in MCPLL is filtered out over a wider range and because of that this architecture exhibits considerably lower jitter generation, comparable to performance of LV VCO based PLLs.

Data-Driven Phase Comparator

In some embodiments, the above-described matrix phase comparator architecture may be applied to extraction of a clock signal from transitions occurring on multiple data sub-channels, as one example on the various sub-channels of a vector signaling code communicated over a multi-wire bus. In such embodiments, vector signaling codes with guaranteed transition density over time resulting from bit-level encoding, or provided by vector encoding schemes such as taught by [Shokrollahi I] are amenable to such combination. FIG. 25 illustrates a receiver utilizing a data-driven phase comparator, in which data signals received in parallel from MIC0-MIC4 are monitored for transitions, providing clock phase information to Clock Recovery 2600 and thus controlling the phase of sampling clocks ph000, ph090, ph180, ph270. FIG. 26 provides a more detailed view of Clock Recovery 2600, which utilizes matrix phase comparator 2610.

Considering the multiple data inputs being monitored by such a system, several operational considerations are apparent. First, as any received data bit may remain in either the “1” or “0” state in consecutive unit intervals, only data transitions between those states are relevant to PLL phase. In particular, between any two consecutive unit intervals a transition may or may not occur in any given data bit; indeed, no transition may occur on any data bit in a given clock interval. If a transition does occur, the matrix phase comparator may utilize the timing of that transition to update the PLL clock phase, while if no transition occurs, the PLL clock may be allowed to continue unchanged. If two or more data lines transition in the same clock interval, the timing errors derived from each such transition may be summed, which is consistent with the previously-described matrix phase comparator behavior.

This behavior suggests that a state-machine phase detector may be a suitable candidate for the phase comparison elements of the comparison matrix, as such designs may be configured to respond only to signal transitions rather than signal level, and may be configured to output a “no change” result in the absence of a signal transition. In some embodiments, the partial phase comparators 2712 may take the form of edge-triggered bang-bang detectors configured to generate partial phase-error signals in response to determining a transition occurred. A block diagram of an exemplary edge-triggered bang-bang phase detector is shown in FIG. 29. In some embodiments, the partial phase comparators may take the form of linear edge-triggered phase detectors to generate partial phase-error signals in response to determining a transition occurred. A block diagram of a linear edge-triggered phase detector is illustrated in FIG. 30, and a corresponding waveform is shown in FIG. 31.

Another embodiment may incorporate a data signal transition detectors, one example including an XOR gate comparing a data signal with a slightly time delayed copy of the same data signal, for example after passing through a logic buffer gate. Such an embodiment will output a logical pulse at each transition, and the edge of such pulse may be phase compared to a PLL clock edge, using any phase detector as previously described. An advanced embodiment may further incorporate a gating or time windowing function in the partial phase detectors to produce a “no change” error result from any partial phase detector not receiving a data signal transition in a given time interval.

FIG. 26 illustrates one embodiment of a multi-line Clock Recovery 2600. MIC0-MIC4 are the detected vector signaling code sub-channels (i.e. decoded data bits) for a code similar to that used in the example of FIG. 2. Any transition on a detected vector signaling code sub-channel may produce a partial phase-error signal relative to one or more local clock phases, each such partial phase-error signal being (in this example) an analog signal either pulsing up, pulsing down, or remaining unchanged. The summation 2650 of all partial phase-error signals is low pass filtered 2660, with the result used to adjust the VCO 2670 frequency. In some embodiments, if the transmission medium has significantly different propagation velocity for different propagation modes, various sub-channels may experience eye-closures, resulting in inter-subchannel skew. In order to offset such inter-subchannel skew, the phase interpolator 2690 may be configured to independently adjust phases of each local oscillator signal according to the associated data signal in order to correct for such sub-channel specific skew. Alternatively, analog delay elements (not shown) may be used to introduce sub-channel specific delay to compensate for sub-channel specific skew.

FIG. 27 further illustrates one embodiment of Matrix Phase Detector 2610, as including an array of component phase detectors 2710. As one example not implying limitation, each partial phase detector 2710 may in turn include an edge-sensitive state machine phase detector, charge pump, and configurable weighting function, as described in [Tajalli IV]. Matrix phase detector 2610 thus compares each of detected data signals MIC0-MIC4 to four phases of local PLL clock ph0, ph090, ph180, ph270, with each such comparison 2712 producing a phase comparison result that is subsequently weighted 2715. In one particular embodiment, the weighted results are produced as analog currents, thus all such results 2720, 2730, 2740, 2750, 2760 may be summed by mutual connection at a current summation node 2650, producing aggregate result 2655 directly. As will be clear to one familiar with the art, comparable results may also be produced by explicit summation of voltages, numerical computation, etc. thus no limitation is implied. In some embodiments, partial phase comparator 2710 may further receive the Transition_EN enable signal (not shown) to selectively output the corresponding partial phase error signal. In some embodiments, each partial phase comparator 2710 receives a corresponding Transition_EN(m) signal associated with the corresponding detected data signal received at the partial phase comparator 2710. For example, MIC0 may have an associated enable signal Transition_EN0, MIC1 may have an associated enable signal Transition_EN1, etc.

The weights of the individual matrix comparison elements 2710 of matrix phase comparator 2610 may be set uniformly (i.e. with a transition on any data signal equally affecting all clock phases,) or non-uniformly such that particular clock phases are more or less affected. As described in [Tajalli IV], other effects including simulation of phase offsets, introduction of loop time domain zeroes, etc. may be obtained by selective configuration of matrix weighting factors.

FIG. 26 shows one embodiment of multi-line Clock Recovery 2600. MIC0-MIC4 are the detected vector signaling code sub-channels (i.e. detected data signals or decoded data bits after slicing and sampling) for a code similar to that used in the example of FIG. 2. Any transition on a detected data signal may produce a phase error relative to one or more local clock phases, each such phase error being (in this example) an analog signal either pulsing up, pulsing down, or remaining unchanged. The summation 2650 of all phase errors is low pass filtered 2660, with the result used to adjust the VCO 2670 frequency.

FIG. 28 is a flowchart of a method 2800, in accordance with some embodiments. As shown, method 2800 includes receiving 2802, at a data-driven phase comparator circuit, a plurality of data signals in parallel from a plurality of multi-input comparators (MICs) connected to a multi-wire bus, wherein at least one MIC is connected to at least three wires of the multi-wire bus, and one or more phases of a local oscillator signal, the data-driven phase comparator circuit comprising a plurality of partial phase comparators, generating 2804, a plurality of partial phase-error signals using the partial phase comparators, each partial phase-error signal generated by receiving (i) a corresponding phase of the local oscillator signal and (ii) a corresponding data signal of the plurality of data signals and responsive to a determination that a transition occurred in the corresponding data signal, generating the partial phase-error signal based on a comparison of the corresponding phase of the local oscillator signal and the corresponding data signal, and generating 2806 a composite phase-error signal by summing the plurality of partial phase error signals, the composite phase-error signal for setting a local oscillator generating the one or more phases of the local oscillator signal in a lock condition.

In some embodiments, the partial phase-error signals are analog signals formed using respective charge pump circuits. In such embodiments, the method further includes filtering the composite phase-error signal.

In some embodiments, the method further includes introducing, for a given partial phase comparator, a sub-channel specific delay into the corresponding phase of the local oscillator signal, the sub-channel specific delay associated with the data signal received at the given partial phase comparator.

In some embodiments, the comparison of the corresponding phase of the local oscillator signal and the corresponding data signal is formed using a linear edge-triggered phase detector. Alternatively, the comparison of the corresponding phase of the local oscillator signal and the corresponding data signal may be formed using an edge-triggered bang-bang phase detector.

In some embodiments, the method further includes applying a weight to the partial phase-error signal. In some embodiments, the plurality of data signals have a collective transition density above a predetermined threshold. In some embodiments, the method further includes outputting a no-change result in response to determining no transition occurred. In such embodiments, outputting the no-change result includes setting the partial phase comparator in a high-impedance state.

PLL Startup

In many communications environments, the minimum transition density of data signals may be low, thus for relatively long periods of time the PLL oscillator must continue running with no change in frequency. As previously described, selection of a phase detector design capable of being configured to produce a “no change” output in the absence of data transitions helps satisfy this requirement, as does selection of an oscillator with good stability characteristics.

At startup, however, the situation may be quite different. The VCO may start oscillation at very high or very low frequency relative to its desired operating frequency, and the density of transitions received on data lines may be very high, especially if an initialization procedure transmits training patterns or other special data sequence as part of CTLE adjustment or other receiver calibration. Thus, it is possible that PLL lock may take considerable time, or may result in VCO operation at the wrong frequency.

[Tajalli III] describes a “Frequency Lock Assist” for PLL initialization, consisting of an additional phase/frequency detector that can override spurious frequency excursions and force the PLL into a normal operational mode, at which point its normal phase comparator may take over.

Alternative Embodiments

The clock signal received from MIC5 in FIG. 2 after being transported over two dedicated clock wires could just as easily be received from, as one example MIC4, having been transported as one sub-channel of the vector signaling code also carrying the data. This method of embedding the clock in a vector signaling code sub-channel is described in [Shokrollahi II] and [Holden III]. All of the described clock embedding embodiments therein may be beneficially combined with the PLL and timing control mechanisms described herein, without limitation. 

I claim:
 1. An apparatus comprising: a matrix phase comparator configured to obtain a plurality of phases of a reference clock signal and a plurality of phases of a local oscillator signal, and to generate at least two sets of weighted phase-error signals, each respective set of weighted phase-error signals (i) having an associated fixed phase offset and (ii) wherein each weighted phase-error signal of the respective set of weighted phase-error signals corresponds to a comparison of a respective phase of the reference clock signal to a corresponding phase of the local oscillator signal having the fixed phase offset relative to the respective phase of the reference clock signal; and a summation circuit configured to receive the weighted phase-error signals of each of the at least two sets of weighted phase-error signals and to responsively generate a composite interpolated phase-error signal by forming a summation of the weighted phase-error signals, the composite interpolated phase-error signal provided to a local oscillator generating the plurality of phases of the local oscillator signal.
 2. The apparatus of claim 1, wherein each weighted phase-error signal is generated using a dynamically-weighted exclusive OR (XOR) phase comparator.
 3. The apparatus of claim 2, wherein each weighted phase-error signal comprises a plurality of weighted segments, each weighted segment generated by enabling a corresponding logic branch of the dynamically-weighted XOR phase comparator responsive of a respective input logic combination of a given phase of the reference clock signal and the corresponding phase of the local oscillator signal.
 4. The apparatus of claim 1, wherein each weighted phase-error signal is generated by a respective exclusive OR (XOR) phase comparator.
 5. The apparatus of claim 4, further comprising a respective weighting circuit connected to an output of the respective XOR phase comparator.
 6. The apparatus of claim 1, wherein each weighted phase-error signal in a given set of weighted phase-error signals has a uniform weight.
 7. The apparatus of claim 1, wherein at least two weighted phase-error signals in a given set of weighted phase-error signals of the at least two sets of weighted phase-error signals have different weights.
 8. The apparatus of claim 1, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 45-degrees. degrees.
 9. The apparatus of claim 1, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 90-degrees.
 10. The apparatus of claim 1, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 0-degree.
 11. A method comprising: obtaining, at a matrix phase comparator, a plurality of phases of a reference clock signal and a plurality of phases of a local oscillator signal; generating, using the matrix phase comparator, at least two sets of weighted phase-error signals, each respective set of weighted phase-error signals (i) having an associated fixed phase offset and (ii) wherein each weighted phase-error signal of the respective set of weighted phase-error signals corresponds to a comparison of a respective phase of the reference clock signal to a corresponding phase of the local oscillator signal having the fixed phase offset relative to the respective phase of the reference clock signal; generating, using a summation circuit, a composite interpolated phase-error signal by forming a summation of the weighted phase-error signals; and providing the composite interpolated phase-error signal to a local oscillator generating the plurality of phases of the local oscillator signal.
 12. The method of claim 11, wherein each weighted phase-error signal is generated using a dynamically-weighted exclusive OR (XOR) phase comparator receiving a phase of the reference clock signal and the corresponding phase of the local oscillator signal.
 13. The method of claim 12, wherein generating each weighted phase-error signal comprises generating a plurality of weighted segments, each weighted segment generated by enabling a corresponding logic branch of the dynamically-weighted XOR phase comparator responsive of a respective input logic combination of a given phase of the reference clock signal and the corresponding phase of the local oscillator signal.
 14. The method of claim 11, wherein generating each weighted phase-error signal comprises forming an exclusive OR (XOR) comparison of a phase of the reference clock signal and the corresponding phase of the local oscillator signal.
 15. The method of claim 14, wherein generating each weighted phase-error signal further comprises applying a weighting factor to the XOR comparison of the phase of the reference clock signal and the corresponding phase of the local oscillator signal.
 16. The method of claim 11, wherein each weighted phase-error signal in a given set of weighted phase-error signals has a uniform weight.
 17. The method of claim 11, wherein at least two weighted phase-error signals in a given set of weighted phase-error signals of the at least two sets of weighted phase-error signals have different weights.
 18. The method of claim 11, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 45-degrees.
 19. The method of claim 11, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 90-degrees.
 20. The method of claim 11, wherein one set of weighted phase-error signals of the at least two sets of weighted phase-error signals has an associated fixed phase offset of 0-degree. 